1. Field of the Invention
The present invention is directed to a process and apparatus for mechanically removing, by scraping, brushing, sand-blasting or grit-blasting, a coating of low thermal conductivity and of low thermal effusivity, which in addition, is tacky at ambient and above ambient temperature conditions, bonded to a substrate of much higher heat conductivity and much higher thermal effusivity, by first refrigerating the coating in order to render it less tacky and even brittle before mechanical removal. Although the principle of the invention is not so limited, the present invention is directed to dielectric coatings, of organic or non-organic nature bonded onto a metallic substrate, for example organic coatings such as hot or cold applied coal tars, coal tar epoxies, asphalt, polyethylene phenolic baked epoxies, amine cured epoxies or polyvinyl chloroacetates, any of those coatings optionally incorporating inorganic films or fabrics. Furthermore, and more specifically, the present invention is directed to processes for the continuous embrittlement and mechanical removal of outer annular protective coatings such as, but not limited to, coal tar bonded to, an annular steel substrate such as, but not limited to an oil or gas transmission pipeline.
2. Description of the Related Art
Refrigeration apparatuses using cryogenic liquid spray heat transfer are disclosed in U.S. Pat. No. 4,956,042 to Hubert et al and in U.S. Pat. No. 4,963,205, the subject matter of both of which is hereby incorporated by reference. Those applications disclose pipeline traveling liquid nitrogen (LN.sub.2) spraying refrigeration tunnels which enable pipeline rehabilitation operations to proceed faster and with complete success in removing a coating and its primer from a pipe or a pipeline, thereby allowing the unimpaired inspection of the pipe for the detection of dangerous corrosion pits and, if necessary, the selection of pipe sections that need to be replaced, in addition to providing a pipe surface of adequate characteristics (first, cleanness, i.e., absence of old coating, old primer, corrosion spots, and second, rugosity) for repriming/recoating (either after scraping alone or in combination with brushing and/or grit- or sandblasting depending on the new coating to be applied and on its required anchor pattern depth).
The process and apparatus described in Hubert et al emphasizes the simplicity of the LN.sub.2 tunnel, its incorporation into the typical pipeline traveling equipment and its high speed of refrigeration. The process and apparatus described in U.S. Pat. No. 4,963,205 emphasizes a different design of the LN.sub.2 tunnel which results in lower LN.sub.2 consumptions, in higher refrigeration efficiencies, in more uniform circumferential refrigeration fields and in high refrigeration speeds compared to the apparatus described in Hubert et al, together with a control/safety/monitoring system for said tunnel.
However, the process and apparatus disclosed in these applications were invented at a time in which typical acceptable pipeline traveling speeds were of the order of 6 feet/min, and speeds of 12 feet/min were considered exceptional. The magnitude of the capital and labor assets immobilized during a pipeline rehabilitation job and the increasing frequency of pipeline rehabilitation jobs due to the aging of the North American and Canadian transmission pipelines to and beyond their expected lifetime, and due to increasing concerns about the safety of older pipelines, have started a new trend in the pipeline industry. Pipeline contractors need to complete the jobs faster. In 1987, 3000 linear feet/day were the norm. Currently, the pipeline industry specifies 7,000 and even 10,000 linear feet/day. Despite their high refrigeration rates, the tunnels of the length disclosed in these applications would not be able to achieve those daily processing rates. Said tunnels could, of course, be lengthened in order to provide the same refrigeration dwell time while traveling faster. However, such an increase in length would generate equipment handling problems, equipment structural integrity problems, equipment driving force problems and problems in the travel of the tunnel around pipe bends.
The above problem is further compounded by the varying thicknesses of outer protective coatings that were applied on the pipelines. Bituminous coatings such as asphalt or coal tar coatings, especially when gravity fed during the initial coating operation, can have thicknesses well in excess of the 60 mils thickness that was implicitly assumed as the norm for bituminous pipeline coatings, and even in excess of the 120 mils thickness that was implicitly assumed as an extreme condition for bituminous pipeline coatings in the above-mentioned applications. Since pipeline outer protective coatings are dielectric in nature (minimum test voltage for a 62 mils thick coal tar coating is 9,800 volts) and resist water penetration, they usually are also good heat insulators (coal tar heat conductivity is about 0.15 W/mK compared to 0.02 W/mK for polyurethane foam insulation and compared to 60 W/mK for carbon steel). Hence, the thicker the coating, the slower the transmission of cold will be from the outer surface of the coating to the steel/coating interface. Especially at larger thicknesses, the coating's heat conduction becomes the process limiting factor, as will be shown in a numerical simulation derived figure. Since the coating must be embrittled through its entire thickness to allow for successful mechanical removal, the operation speed of a tunnel of given length will decrease sharply as the coating thickness increases. Furthermore, since the amount of sprayed cryogen per unit time remains the same, the consumption per linear foot increases accordingly, and since the overall heat removal from steel and coating remains roughly the same, the process efficiency decreases accordingly.
To maintain an admissible operating speed, the tunnels of the above-mentioned applications need to be lengthened, which generates the above mentioned problems and larger capital costs. Lengthening those tunnels would, moreover, not alter the high specific cryogen consumption and the resulting low efficiency, thereby generating high operating costs.